In many practical applications, it is desired to transmit signals between two electrical circuits that are electrically isolated from each other. Due to a desire for small device size and complete electrical isolation between the circuits, four terminal devices, commonly called opto-isolators, have been developed. These devices utilize optical coupling, rather than the electrical coupling used in transformers and relays, to link the two electrical circuits. Opto-isolators use a light source, commonly a light emitting diode (LED), located in the electrical input circit and a photodetector, located in the electrical output circuit and optically coupled to the light source, to couple the two electrical circuits. Current flowing in the input circuit causes the LED to emit light, and some of this light is received by the photodetector and causes an electrical current to flow in the output circuit. It should be understood that the term, "light", as used in this specification, refers to electromagnetic radiation in both the visible and infrared regions.
Opto-isolators typically use discrete devices, that is, the light source and photodetectors are manufactured separately and individually positioned in an optical cavity to form the opto-isolator. The light source is connected to two input terminals, and the photodetector is connected to two output terminals. The light source and photodetector are generally formed from different materials. For example, opto-isolators commonly used today have LEDs made from Group III-V compounds, such as GaAs, GaP, GaAs.sub.1-x P.sub.x and Ga.sub.1-x Al.sub.x As, and Si photodetectors. The presence of discrete devices means that considerable care has to be exercised in positioning the LED and the photodetector, both with respect to each other and the cavity, to obtain efficient light coupling. Additionally, cavity construction and the material used to form the cavity are often critical.
For reasons of manufacturing economics, as well as efficient coupling of light between the light source and detector, a monolithic or integrated opto-isolator would be desirable. Such a device would be fabricated on a single semiconductor chip from a single semiconductor material, i.e., both the light source and light detector would consist of the same semiconductor material. The use of a single material and a single chip affords the possibility of simplicity of fabrication as positioning of the LED and photodetector with respect to each other may be accomplished automatically. The automatic positioning of LED and photodetector with respect to each other may reduce optical losses due to misalignment of LED and photodetector.
Fabrication of a monolithic opto-isolator has been attempted. For example, U.S. Pat. No. 3,705,309 discloses an opto-isolator using a thin film to optically couple an electroluminescent region and a photoconductive region. One embodiment of the device uses an optically conducting semiconducting film with the light generating and light detecting regions produced in the film by two separate diffusion steps. Metal electrodes are attached to the thin film and permit appropriate biasing of both the light generating and light detecting regions. The thin film may be made from a single Group II-VI or Group III-V semiconductor material such as zinc sulfide or gallium arsenide.
While perfectly adequate for some uses, the described and similar devices have drawbacks which are undesirable and limit the number of situations in which they can be used successfully. One drawback of the described device arises because the region between the elecroluminescent and photoconductive regions is electrically conducting and, as a result, only relatively small differences in potential between the electrode pairs may be tolerated before the device breaks down. Another drawback arises because some light is lost, i.e., not received by the photoconductive region, both to the left and through the top of the device because of the position of the electroluminescent junction. There is only one photoconductive region, and the electrode size cannot be increased to reflect light back into the thin film and, thus, reduce light losses through the top of the device, because the electrode must remain electrically isolated from the other conductivity regions. The coupling efficiency between light source and light detector of the device is thus limited. Finally, the thin film, including both the light source and light detector, in the embodiment described is made from the same material, and the semiconductor material forming the intermediate region between the light emitter and detector has a bandgap equal to that of the emitting region. As a result, the intermediate region is an efficient absorber of the emitted light, and the device coupling efficiency is again decreased.